The catalytic combustion of gases such as methane (CH.sub.4) is typically accomplished with a metallic resistance-type heating element that heats a catalyst that is typically covered by a layer of material to protect it, for example, from sulfur dioxide. The heating element is normally made of a noble metal such as platinum or gold and the catalyst is from group VII, that is usually a palladium, platinum, rhodium, or iridium alloy.
As described in British patent document No. 2,083,630 it is known to use a pellistor as sensor in a process which cataytically burns methane in an oxygen-containing gas, normally air. The catalytic combustion of methane generates temperatures of about 800.degree. K. Other hydrocarbons such as butane or propane are typically combusted at lower temperatures of around 500.degree. K. The catalytic combustion can serve to rid an exhaust gas of a harmful component, or can be used merely as will be described below in a laboratory or industry setting to determine the concentration of the hydrocarbon gas at a location, for instance in a process stage or smokestack.
The resistance-type electrical heater heats the catalyst material so as to trigger the catalytic combustion. This heater serves simultaneously to detect whether catalytic combustion is taking place and simultaneously therefore determines whether any methane or other gas being detected is present. In the absence of any combustion the temperature of the heater and catalyst will remain uniform, presuming uniform energization of the heater and uniform throughput of gas. When methane enters the system, however, catalytic combustion takes place and the temperature of the catalyst and of the heater increases due to this secondary input of heat energy from the exothermic catalytic combustion. As the temperature of the resistive heater increases so does its resistance, causing a corresponding and measurable decrease in current flow, once again presuming unchanging energization voltage. The change in resistance in ohms or the change in current in amperes therefore is fairly directly proportional to the concentration of the gas being catalytically combusted, making it possible to accurately measure such concentration.
A standard platinum wire of the type used in such a sensor has, unfortunately, a very small change in resistance when the temperature changes by, say, 0.003/.degree.K., as can occur when a quantity of methane appears that should be measured. Gold and palladium, which are also usable as resistance-type heating elements in burners for methane and similar hydrocarbon gases, also do not change in resistance greatly with small temperature changes. Thus it is necessary to use complicated and expensive monitoring equipment capable of sensing tiny variations in resistance and converting them into usable gas-concentration readings.
Such devices therefore are quite costly to manufacture. They are also fairly delicate because the wires are made very thin to spare the costly noble metals that must be used for the necessary catalytic reaction. Finally they typically operate sluggishly, responding slowly because it takes some time for the exothermic catalytic combustion to heat up the thermal mass of the heater and catalytic body carried on it.